1. Field of the Invention
This invention relates to geophysical surveying, and more particularly to techniques for surveying terrain resistivity.
2. Review of the Art
Various techniques have been utilized for the detection of subsurface geological anomalies, characterised by different resistivity from surrounding terrain. One such technique relies upon the detection of secondary signals produced as a result of exposure of terrain being mapped to the coherent electromagnetic field produced by a distant VLF radio station. Some what analogous techniques utilize non-coherent sources such as naturally occurring magneto-telluric and audio frequency magneto-telluric electromagnetic fields.
Available VLF electro-magnetic fields have been used for over twenty years to map subsurface geology. In the one mode of implementation the horizontal (primary) VLF magnetic field component is used as a phase and amplitude reference against which to measure the normalized in-phase and quadrature phase components of the secondary vertical VLF magnetic field. In some cases the tilt angle (or tangent of the tilt angle, usually expressed as a percentage) and elipticity of the polarization ellipse is measured instead, but in the usual case where the anomalous or secondary VLF magnetic field components are relatively small compared with the primary field this measurement technique also yields quantities which are simply related to the in-phase and quadrature phase components of the vertical magnetic field.
In the early days of the VLF technique, survey interpretation was based on the assumption that, even in an electrically conductive earth, the time-varying primary VLF magnetic field induced vortex or eddy currents in vertical conductors. These vortex currents generated the secondary VLF magnetic field which was used to locate and characterise the conductor, using well established but flawed principles governing the ratio and phase of the secondary field components as functions of target size, depth and conductivity. Since only targets with appreciable vertical extent would be excited by the horizontal primary magnetic field the expected (and indeed measured) anomalies often took the form of a lateral S-curve with a zero-crossing. The location of the target was under the zero-crossing or, in the case where the response from nearby targets was also strong enough to contribute to the total response, the location was under the point of maximum inflection of the central limb of the curve.
Such multiple-target responses occur frequently and the resultant interference between the anomalies sometimes makes survey data interpretation quite difficult. In order to rectify this situation Fraser ("Contouring of VLF-EM data", Geophysics 34, page 958-967) proposed applying a simple numerical filter to the survey data which would simultaneously
(1) numerically differentiate the data horizontally so as to transform the cross-overs into peaks, which could then be contoured.
(2) high-pass filter the data so as to attenuate the response from slowly varying anomalies and relatively enhance the response from localized anomalies.
(3) not emphasize random geological noise.
(4) be simple to apply for untrained field personnel.
The resultant "Fraser filter" has been widely used in VLF data processing. Examples of its application will be considered hereinafter.
More recently Karous and Hjelt ("Linear filtering of VLF dip-angle measurements", Geophysical Prospecting 31 (1983), pages 782-794) proposed a similar numerical filter, the output of which is interpreted in terms of the equivalent current density at a specified depth that would cause the measured VLF magnetic field strength. This filter also gives contourable anomaly peaks rather than cross-overs.
Finally Lankston and Hecker ("Enchancing VLF-EM data through application of frequency domain operators", Proc. 2nd National Outdoor Action Conference on Aquifer Restoration, Groundwater Monitoring and Geophysical Methods, Las Vegas (1988), pages 655-673) suggest use of the Hilbert transform to improve the geological signal-to-noise ratio over that achieved by the Fraser filter, but at the expense of greater mathematical complexity.
From the above it will be noted that use of numerical data processing (i.e. filtering) to enhance VLF data and simplify interpretation is a common practice.